The present invention relates to a system for separating matter from a support liquid in which it is suspended.
For example, the system of the invention can be used for separating waste paint solids from an aqueous medium which has been used to wash paint solids from the air exhausted from a paint spray booth.
U.K. Pat. No. 1,540,723 describes apparatus for accumulating waste paint solids which generally comprises a floatation tank having an inlet near its base for a liquid medium, for example, an aqueous solution, containing the paint solids in suspension. The liquid with the paint solids is continuously pumped into the tank through this inlet and is continuously removed through an outlet provided in the tank. The rate of entry and removal of liquid is controlled so that a head of liquid is formed in the tank above the inlet and outlet and liquid flowing through the tank is permitted a sufficient residence time within the tank to enable paint solids to float upwardly to the surface, where they accumulate and form a raft. This raft of accumulated paint solids is removed from time to time by causing the raft to pass over a weir provided at one side of the tank.
The disclosure in U.K. Pat. No. 1,540,723 suggests that sonic detectors might be provided to determine the level of the raft of accumulated paint solids within the vessel.
Several features of the system and its operation assist in the formation of the paint raft. Thus, if the liquid is aerated, bubbles of air will attach to the paint solids and thus ensure that they float more readily. Furthermore, the inlet is generally upwardly directed so that the incoming stream of liquid is directed towards the surface of the liquid within the tank. In addition, the velocities of the liquid flow throughout the system are arranged such that the paint solids are encouraged to rise to the surface and form the raft without re entrainment and downward movement.
The applicants have now found that a system as described in the above identified UK Patent operates most effectively and efficiently if the significant liquid levels can be maintained at predetermined values even if the system is subjected to changes which cause alteration in these levels. Thus, changes in the aeration of the liquid, for example by adding chemicals thereto, changes the levels. In addition, debris in the system can alter flow rates which leads to changes in the levels. Level changes can also be caused by liquid loss at local points and/or by changes in the temperature of the liquid.
Of course, it is known to prevent the level of liquid in a tank from rising above a predetermined level. Thus, types of float valves are described in U.K. Pat. Nos. 1,327,991 and 1,559,517 and are arranged to open a drain passage when the liquid in the tank rises to the predetermined level. The liquid then drains away. The cycle is repeated when liquid arriving subsequently into the tank also attains the predetermined level. UK Pat. Nos. 1,604,795, 2,065,336B and 2,107,204 also show systems preventing liquid rising above a predetermined level, but in these systems the float valves are replaced by sensors in contact with the liquid arranged to actuate valves on the drain passages. None of these earlier systems is able to maintain a liquid constantly at a predetermined level.